Article Figures & Data

Figures

Electrophysiological properties of plateau potentials. A, Confocal image of a transverse section of the lumbar spinal cord shows motoneurons retrogradely labeled with the fluorescein-conjugated cholera toxin B subunit tracer injected into the triceps surae muscle. Scale bar, 100 μm. B, Bistable motoneuron displaying a plateau potential (black traces) when the depolarizing current pulse was increased either in amplitude (B1) or in duration (B2) or when the holding membrane potential was depolarized (B3). Current pulses in B2 and B3 were given from a depolarized membrane potential using bias current. Gray traces show the sADP outlasting the current pulse when the voltage threshold to trigger the plateau potential was not reached. C, Firing properties of a non-bistable motoneuron unable to generate a plateau potential in response to a 2 s depolarizing current pulse. C, Inset, Illustrates with expanded time scale the typical pronounced afterhyperpolarization (AHP) that follows the current pulse (action potentials are truncated). B, C, Instantaneous frequency plots are shown on top of intracellular recordings. D, Proportions of bistable and non-bistable motoneurons at the beginning and the end of the first postnatal week. E, Plateau potential triggered by a brief depolarizing current pulse in the presence of CNQX (10 μm), AP5 (50 μm), strychnine (5 μm) and bicuculline (20 μm). F, Electrical stimulation of the dorsal root (10 Hz, 2 s) induced plateau potential, which was stopped by hyperpolarization. In this and the following figures, bottom traces are the injected current.

Electrophysiological properties of the sADP. A, Superimposed voltage traces from a motoneuron showing the sADP in response to current steps of increasing amplitude (A1) or duration (A2). A, Insets, The linear relationship between the area of the sADP and the number of spikes during the stimulus. B, Voltage traces illustrating the wind-up of the sADP in response to repetitive depolarizing current pulses of constant amplitude applied either at resting membrane potential (B1) or at a more depolarized potential producing a plateau potential (B2). C, Superimposed voltage traces from a bistable motoneuron recorded at resting membrane potential illustrating the switch of the sADP (gray trace) into a plateau potential (black trace) when the initial 2 s square pulse was followed by a sequence of suprathreshold depolarizing current pulses.

Pharmacological profile of plateau potentials. A1–A3: Effect of blocking the L-type Ca2+ channels by nifedipine (20 μm) on plateau potentials. B, Effect of the progressive blockade of the persistent sodium current by 5 μm riluzole (B1) or 10 nm TTX (B2) on plateau potentials. C, Facilitation of plateau potentials by upregulating the persistent sodium current with veratridine (40 nm). Motoneurons (Mns) were recorded at a holding potential between −60 and −55 mV.

The sADP requires spiking-dependent Ca2+ influx. A, Superimposed voltage traces in response to a 2 s depolarizing pulse before (black trace) and after (gray trace) TTX application (1 μm). The graph shows the mean amplitude of the sADP before and after drug application. B, Superimposed voltage traces collected under TTX, in response to a 2 s depolarizing pulse on which additional brief depolarizing pulses that mimic action potentials were (black trace) or were not (gray trace) applied. C, Superimposed voltage traces in response to a 2 s depolarizing pulse, collected under TTX before and after adding TEA (10 mm). D, Superimposed voltage traces in response to a 2 s depolarizing pulse, collected under TTX and TEA before and after the removal of extracellular Ca2+. Graphs show the mean amplitude of the sADP before and after removing Ca2+. Note that action potentials were truncated for the visualization of the sADP. Holding potential, −60 mV. Error bars indicate SEM. ns, Not significant; **p < 0.01, ***p < 0.001 (Wilcoxon paired test).

The sADP and plateau potentials are dependent on ICaN that uses Na+ as the main charge carrier, not a Na+/Ca2+ exchanger. A, Superimposed voltage traces in response to a 2 s depolarizing pulse before (black traces) and after (gray traces) chelating the intracellular Ca2+ with BAPTA (10 mm) in the pipette solution. B, Left, Superimposed current traces from a motoneuron recorded under TEA (10 mm), TTX (1 μm), and apamin (100 nm), held at −60 mV, step-depolarized to +10 mV and then returned to potentials between −60 and 0 mV. On the right, I–V relationship of the peak inward tail current plotted against the return potential before (black trace) and after (gray trace) lowering the extracellular concentration of Na+. C, D, Superimposed current (C) and voltage (D) traces in response to a 2 s depolarizing pulse before (black traces) and after (gray traces) lowering the extracellular concentration of Na+. E, Superimposed voltage traces from a bistable motoneuron recorded at resting membrane potential in response to a 2 s square pulse followed by a series of brief depolarizing current pulses before (black trace) and after (gray trace) lowering the concentration of Na+. F, Superimposed voltage traces in response to a 2 s depolarizing pulse before (black traces) and after (gray traces) the blockade of the Na+/Ca2+ exchanger by lithium. A, C, D, F, Right, Graphs show the mean amplitude of the sADP before and after the drug application. Recordings were made under TTX (1 μm) and TEA (10 mm). The apamin (100 nm) was superfused for voltage-clamp recordings. Holding potential, −60 mV. Error bars indicate SEM. ns, Not significant; *p < 0.05, **p < 0.01 (Wilcoxon paired test).

A schematic ménage à trois relationship between currents underlying the different phases of the plateau firing mode. The Ca2+ entry via voltage-dependent Ca2+ current occurs during the train of action potential (step 1 and 2 in A and B). This increase in intracellular Ca2+ triggers a voltage-independent cation current that depolarizes the membrane and mediates the sADP (step 3 in A and B). The positive change in membrane potential opens voltage-dependent persistent Na+ current to maintain a train of action potential (step 4 in A and B). The repetitive spiking activity will then induce Ca2+ entry via voltage-dependent Ca2+ current and so forth.